Analog-to-digital (A/D) converters are commonly used in microelectronics systems to convert real-world analog signals into digital signals that can be processed by digital systems such as computers digital audio and video systems and wireless communication systems. A/D converters may be implemented as standalone semiconductor devices, or integrated along with other circuitry on a single integrated circuit. While a today, converters may be implemented using a number of different architectures, the successive approximation A/D converter is widely used in applications that require medium speed and resolution, for example, measuring multiple DC inputs in a test or measurement system, or performing an A/D conversion of an audio signal.
A typical successive approximation A/D converter converts an analog signal to a digital signal by comparing various output values of digital to analog (D/A) converter with the analog input signal over the course of a number of clock cycles. For example, in a first conversion cycle the most significant bit (MSB) is determined by comparing the analog input signal with the mid-scale output of the D/A converter. If the analog input value exceeds the midscale output of the D/A converter, the MSB is set to a logical one. If, on the other hand, the analog input value is less than the midscale output of the D/A converter, the MSB is set to a logical zero. During the next conversion cycle, the analog input signal is compared with the ¼ scale or ¾ scale output of the D/A converter depending on the outcome of the first conversion cycle, and the second most significant bit is determined. Each conversion cycle progresses based on the previously determined bit until every bit in the digital output word is determined.